1. Field of the Invention
The present invention relates to diagnostic imaging and more particularly, to a radiographic X-ray detector and methods of making them.
2. Description of the Related Art
X-ray imaging is a non-invasive technique to capture medical images of patients or animals as well as to inspect the contents of sealed containers, such as luggage, packages, and other parcels. To capture these images, an X-ray beam irradiates an object. The X-rays are then attenuated as they pass through the object. The degree of attenuation varies across the object as a result of variances in the internal composition and/or thickness of the object. The attenuated X-ray beam impinges upon an X-ray detector designed to convert the attenuated beam to a usable shadow image of the internal structure of the object.
Increasingly, radiographic flat panel detectors (RFPDs) are being used to capture images of objects during inspection procedures or of body parts of patients to be analyzed. The RFPDs have a scintillator layer such as CsI:Tl or Gd2O2S which converts X-rays into light which then interacts with an amorphous silicon (a-Si) semiconductor layer, where electric charges are created.
The created electric charges are collected via a switching array, comprising thin film transistors (TFTs). The transistors are switched-on row by row and column by column to read out the signal of the detector. The charges are transformed into voltage, which is converted in a digital number that is stored in a computer file which can be used to generate a softcopy or hardcopy image. Recently Complementary Metal Oxides Semiconductors (CMOS) sensors are becoming important in X-ray imaging. The detectors based on CMOS are already used in mammography, dental, fluoroscopy, cardiology and angiography images. The advantage of using those detectors is a high readout speed and a low electronic noise. Generally, the imaging array including TFTs as switching array and photodiodes is deposited on a substrate of glass.
One way of producing scintillator layers with high image resolution is by vapour deposition of scintillators onto a radiation transparent substrate. Scintillator layers consisting of needle crystallites are particularly suitable to be deposited by vapour deposition. Using thin radiation transparent substrates has the advantage of weight reduction and cost reduction in both Computer Radiography (CR) systems and RFPD's. The cost of thin radiation transparent substrates is lower in comparison with commonly used plates based on metal or reinforced plastic. Additionally in RFPD's a good contact between the scintillator layer and the sensor or imaging array is required to obtain good optical coupling leading to a high image quality. This good optical coupling can be achieved when scintillator layers are deposited on thin radiation transparent substrates which are flexible. The flexibility of the radiation transparent substrate and the flexibility of the scintillator layer guarantee a good contact between the scintillator and the sensor. It is further advantageous to have a thin radiation transparent substrate to minimise the weight of the substrate and hence the weight of radiographic flat panel detector. A low weight is mandatory for the portability of RFPD's. In the medical field, these X-ray detectors have to be carried around by the medical staff and any weight reduction is, therefore, beneficial for the user of the detector. Another advantage of using thin radiation transparent substrates in the production of scintillators for radiographic flat panel detectors is the reduced absorption of X-rays by the substrate with respect to normal or thick substrates since the exposure of the scintillator to radiation is done mostly through the substrate.
The fixation of large and thin radiation transparent substrates during the vapour deposition of scintillator layers and more specifically, needle scintillators is difficult. Indeed, it is then hard to obtain a flat radiation transparent substrate to guarantee a homogeneous layer deposition. Furthermore, it is almost impossible to handle vapour deposited layers on thin and hence flexible radiation transparent substrates without damaging the scintillator layer or without delaminating the scintillator layer from the substrate. This is even more pronounced in case of needle scintillators.
JP2006010616A discloses a manufacturing method of a radiation image transformation panel. In this manufacturing method a layer which contains magnetic material is provided between support holder and the photostimulable phosphor layer. The layer which contains magnetic material is made of ferrite magnets or rubber magnets which are thick layers and which are not very compatible with thin substrates. Thin magnetic films can be applied via the gaseous phase deposition but this is an expensive method and suffers from the same problem related to the difficult handling of thin substrates when thin substrates would be used. Moreover, the presence of magnetic material on the substrate of a scintillator, once built in a RFPD, can be disadvantageous due to the interaction between the magnetic field from the magnetic material and the electronic components of the sensor.
JP2004018938A discloses a deposition method of phosphor layers onto a substrate, using magnetic attraction power between the substrate supporting part and the substrate. This is realised by providing a magnetic layer on the side of the substrate opposite to the side which has to be covered by a phosphor layer. The substrate consists of an inorganic material such as glass or silicon. Due to the high stiffness of these substrates, a good coupling between the scintillator layer and the sensor, a TFT, is very difficult to achieve. Moreover, the presence of magnetic material on the substrate of a scintillator, once built in a RFPD, can be disadvantageous due to the interaction between the magnetic field from the magnetic material and the electronic components of the
JP2006219516A discloses a manufacturing method of a radiological image conversion panel holding a substrate, preferably Aluminium by an electrostatic chucking function of a substrate holder, and vapour-depositing a centre or the whole surface of this substrate. The substrate holder requires still metal fittings to fix the edges of the substrate against the surface of the substrate holder. These metal fittings cause irregular deposition of phosphor layers at the edges of the substrate.
EP1630260 discloses a magnetic latch for securing substrates on a planetary rotating platform suspended above a coating source in a vacuum chamber of vapour deposition systems. The magnetic latch includes a permanent magnet, which is moveable between a latching position, in which the permanent magnet magnetizes temporarily the latch for attracting a substrate holder, and an unlatching position, in which the permanent magnet is connected in a bypass circuit, thereby demagnetizing the latch for releasing the substrate holder.
EP1630261 discloses a partially disposable substrate holder used in magnetic latches for securing substrates on a planetary rotating platform suspended above a coating source in a vacuum chamber of vapour deposition systems. The substrate holder includes a reusable base formed, at least partially, from a ferromagnetic material, which is attracted to the magnetic latch, and a disposable cover formed from a relatively inexpensive, ferromagnetic, easily formable material, which encourages adherence of coating material and has a low vapour pressure at coating temperatures.
WO10023969A describes a radiation image conversion panel which can be transported avoiding contact between the phosphor layer and the rollers of the transport system. On one side of the panel, a ferromagnetic layer is provided to assure a magnetic attraction power with magnetic conveyor rolls. The phosphor layer is applied via a coating from a coating solution containing also a binder. This results in a phosphor layer which is not easily damaged and where good contact with the image sensor can be achieved.
U.S. Pat. No. 2,694,153 discloses an X-ray intensifying screen which has a support which responds to the effect of a magnetic field. This response is used to bring the intensifying screen into intimate and uniform contact with an X-ray sensitive film. The support may contain a layer of finely divided particles of magnetizable material in a film forming binding agent. The layer of magnetizable material is not used in a vapour deposition process during the production of the X-ray intensifying screen.
None of these documents describe a method of vapour depositing a scintillator layer on thin substrates acting as radiation transparent substrates, which guarantees a perfect fixation over the whole surface of the substrate against the substrate holder, without requiring substrate holders having metal latches or movable parts to fix the substrate so as to obtain a good adhesion between the scintillator layer and the radiation transparent substrate and which do not require thick layers with magnetic material onto the substrate. Thin substrates are very preferably required to reduce the absorption of X-rays in a RFPD. It is also desirable to have a method of producing substrates suitable for vapour deposition of layers in an economical efficient way. It is furthermore desired to have a method for producing scintillator layers on radiation transparent substrates which have a low weight and which at the same time can serve as ESD (Electrostatic Discharge) shield in a radiographic flat panel detector.